FBI director Robert Mueller said Wednesday that the nation's top law enforcement bureau uses drones to conduct surveillance on U.S. soil, though only on a "very, very minimal basis."

Mueller, the FBI director since 2001 who is set to retire this year, acknowledged that his agency uses drones in its investigative and law enforcement practices, and is further working to establish better guidelines for the use of drones.

"We are in the early stages of doing that, and I will tell you that our footprint is very small, we have very few, and have limited use. And we're exploring not only the use, but the necessary guidelines for that use," Mueller told senators at a hearing this morning when asked about the use of drones.

FBI Director Robert Mueller testifies about the domestic use of drones during a hearing on Capitol Hill Wednesday.

The government's use of drones on U.S. soil has been well-documented. The Department of Homeland Security, for instance, employs aerial drones to help police the United States border with Mexico.

Mueller said that drones are used for surveillance, though, only on a "seldom" basis.

The FBI director's words come amid a simmering national debate in recent months about what limits should be placed on the government in its law enforcement and anti-terrorism activities.

Sen. Rand Paul, R-Ky., for instance, waged a filibuster challenging President Barack Obama's use of drones in pursuing terrorist suspects. Paul won an affirmation from the administration that it was their thought that it would be illegal for the government to use a drone strike against a U.S. citizen on American soil.

The drones that have come into practice in the United States, though, are different from the armed, militarized drones used in military operations.

Still, the exchange reflects broader concerns about the scope of government power, represented most recently and most vividly by revelations about the National Security Agency's collection of phone and internet "meta-data" for analysis.

Mueller, like virtually every other administration official and senior lawmaker who has spoken about the NSA practices in recent weeks, defended the NSA's activities as an invaluable tool in the government's pursuit of terrorist suspects.

"If we're going to prevent terrorist attacks, we have to be on their communications," Mueller said during his appearance before the Senate Judiciary Committee. "You never know which dot will be critical. You need as many as you can get. Let there be no mistake, there would be fewer dots to connect if you don't have a data base that retains those records."

As to the prosecution of Edward Snowden, the self-admitted leaker of information about NSA monitoring, Mueller said the leak had done legitimate harm to U.S. safety, and vowed to pursue Snowden.

"As to the person who has admitted to making these disclosures, he is the subject of an ongoing criminal investigation," Mueller said. "These disclosures have caused significant harm to our nation and to our safety, and we are taking all necessary steps to hold accountable that person for these disclosures."